#include<bits/stdc++.h>
using namespace std;


int main(){
    int n;
    cin>>n;
    int sum = 0;
    int cnt = 1;
    int day = 0;
    for(int i = 0;i<n;i++){
        sum+=cnt;
        day++;
        if(day % cnt == 0){
            day = 0;
            cnt++;
        }

    }
    cout<<sum<<endl;

    return 0;
}